The present application relates to power conversion, and more specifically to bidirectional current-modulated power converters.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
A new kind of power converter was disclosed in U.S. Pat. No. 7,599,196 entitled “Universal power conversion methods,” which is incorporated by reference into the present application in its entirety. This patent describes a bidirectional (or multidirectional) power converter which pumps power into and out of a link inductor which is shunted by a capacitor.
The switch arrays at the ports are operated to achieve zero-voltage switching by totally isolating the link inductor+capacitor combination at times when its voltage is desired to be changed. (When the inductor+capacitor combination is isolated at such times, the inductor's current will change the voltage of the capacitor, as in a resonant circuit. This can even change the sign of the voltage, without loss of energy.) This architecture has subsequently been referred to as a “current-modulating” or “Power Packet Switching” architecture. Bidirectional power switches are used to provide a full bipolar (reversible) connection from each of multiple lines, at each port, to the rails connected to the link inductor and its capacitor. The basic operation of this architecture is shown, in the context of the three-phase to three-phase example of patent FIG. 1, in the sequence of drawings from patent FIG. 12a to patent FIG. 12j. 
The ports of this converter can be AC or DC, and will normally be bidirectional (at least for AC ports). Individual lines of each port are each connected to a “phase leg,” i.e. a pair of switches which permit that line to be connected to either of two “rails” (i.e. the two conductors which are connected to the two ends of the link inductor). It is important to note that these switches are bidirectional, so that there are four current flows possible in each phase leg: the line can source current to either rail, or can sink current from either rail.
Many different improvements and variations are shown in the basic patent. For example, variable-frequency drive is shown (for controlling a three-phase motor from a three-phase power line), DC and single-phase ports are shown (patent FIG. 21), as well as three- and four-port systems, applications to photovoltaic systems (patent FIG. 23), applications to Hybrid Electric vehicles (patent FIG. 24), applications to power conditioning (patent FIG. 29), half-bridge configurations (patent FIGS. 25 and 26), systems where a transformer is included (to segment the rails, and allow different operating voltages at different ports) (patent FIG. 22), and power combining (patent FIG. 28).
Improvements and modifications of this basic architecture have also been disclosed in U.S. Pat. Nos. 8,391,033, 8,295,069, 8,531,858, and 8,461,718, all of which are hereby incorporated by reference.
The term “converter” has sometimes been used to refer specifically to DC-to-DC converters, as distinct from DC-AC “inverters” and/or AC-AC frequency-changing “cycloconverters.” However, in the present application the word converter is used more generally, to refer to all of these types and more, and especially to converters using a current-modulating or power-packet-switching architecture.
Power conditioning can include different adjustments in power, such as power factor correction, voltage and frequency modifications, and harmonic corrections, among others.
Power factor, a dimensionless number between 0 and 1, is the ratio of real power to apparent power, or the cosine (for pure sine wave for current and voltage) that represents the phase angle between current and voltage waveforms. When both waveforms are not in phase, power factor is less than 1, which is sub-optimal.
Lower power factors draw more current than loads with power factors closer to one for the same amount of power transferred, which increases energy lost in distribution systems, requiring larger transmission and generating equipment to provide for energy needs.
In an electrical power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transmitted. Higher currents can increase the energy lost in the distribution system, requiring larger gauge wiring and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers with a low power factor.
Power Factor Correction (PFC, also referred to as power conditioning) is the process of adjusting those characteristics of electric loads that create a power factor less than 1. Power factor correction can allow for maximally-efficient power distribution.
There are two types of power factor correction: active power factor correction and passive power factor correction. Generally, active power factor correction is preferred, and power factor correction can be accomplished by different techniques, such as: using different capacitors or inductors on output lines; installing synchronous condensers; utilizing active power converters; or other techniques.
A typical switched-mode power supply first rectifies an AC current, forming a DC bus (or DC ripple current) using a bridge rectifier or similar circuit. The output voltage can then derived from this DC bus. Drawbacks can include that the rectifier is a non-linear device, and in such a case, input current is highly non-linear. Non-linear loads from the rectifier can distort the current drawn from the system, and the non-linear input current can exhibit higher energy at harmonics of the frequency of voltage. If these harmonic currents exist at significant levels, overheating and failure of one or more components of a distribution circuit can occur.
These problems can be reduced by increasing the size of components in the distribution circuit, but this also increases costs. Active or passive power factor correction can be used to counteract the distortion and raise the power factor.
Other issues that can affect power quality requiring power factor correction can be variations in voltage and frequency, for which transformers and other devices such as voltage regulators and variable frequency drives can be installed in power converters for regulation. Transformers are generally bulky, and the greater the need for precision, the more expensive the transformers become, reducing the economic viability of this alternative. On the other hand, variable frequency drives can require expensive filters in order to attenuate harmonic levels.
Two of the common nuisances in power grid connections are reactive loads and power harmonics. A switch which turns on or off quickly can introduce voltage spikes onto a power line. Such transients can be introduced, for example, by firing of a thyristor. Moreover, any non-linear load, such as a rectifier or a glow discharge lamp, can introduce harmonics. The commonest power line frequencies (50 Hz or 60 Hz) are extremely low, but their harmonic frequencies (n*50 or n*60 Hz) behave somewhat differently. At the harmonic frequencies, more power will be coupled into parasitic capacitances and their associated resistances. As frequencies get higher, some radiation loss can also occur.
A pure resistor will always pass a current which is proportional to the applied voltage. However, motors and transformers commonly draw current which is out-of-phase with applied AC voltage. Such reactive loads present a problem for power grid design, because (at a given power level) the current drawn by a reactive load will be higher than that drawn by a resistive load. This means that the ohmic losses (I2R) are higher too. This also means that peak load capacity of the distribution network will be lower than it might be otherwise.